Aromatic polyisoimides

ABSTRACT

There are provided polyisoimides having repeating units of the formula ##STR1## wherein R is selected from the group consisting of ##STR2## wherein Me is --CH 3  and wherein X is ##STR3## The polyisoimides of the present invention are soluble in a variety of solvents, including DMAC, THF and the like. 
     The polyisoimides of the present invention are used to produce cured resins having high strength and temperature characteristics as well as low void contents. Curing is effected by heating the polyisoimides at temperatures ranging from about 200° C. to about 370° C.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

BACKGROUND OF THE INVENTION

This invention relates to aromatic polymers. In particular, thisinvention relates to aromatic isoimide polymers.

Considerable research effort has been directed toward the synthesis ofrigid rod polymers for their unique ordering properties that provideextremely high modulus/high strength films and fibers. One class ofmaterials of particular interest is the aromatic heterocyclicbisbenzazole polymers. These polymers exhibit excellent thermal andthermooxidative stabilities. Another class of materials havingcomparable high temperature properties is the aromatic polyimides. Thepolyimides are attractive, not only for their high temperatureproperties, but also because of the low cost of the diamine anddianhydride monomers used in their synthesis.

An aromatic polymide with the desired para-ordered geometry and beprepared from pyromellitic dianhydride (PMDA) and p-phenylene-diamine.High molecular weight polyamic acid has been prepared indimethylacetamide (DMAC) using these monomers; however, thermal orchemical cyclodehydration leads to an insoluble, infusible material.Fabrication of this material is normally carried out via theDMAC-soluble polyamic acid which produces two units of water per repeatunit during high temperature thermal cyclodehydration to the imidestructure. The water produced by this process limits the utility of thismaterial, particularly in the fabrication of thick components.

It is known that certain polyisoimides can be used to form thecorresponding polyimides by thermal curing. Such polyisomides may beprepared by reacting a carboxylic acid dianhydride with a tetravalentaromatic diamine to produce a polyamic acid, and treating the resultingpolyamic acid with a dehydrating agent to produce the correspondingpolyisoimide. The polyisoimide to polyimide route is attractive from thestandpoint that in the course of thermal curing no water vapor isreleased which could cause voids or defects in thick components. Ingeneral, the soluble polyisoimides prepared from aromatic diaminesdisclosed in the prior art are not linear, i.e., that portion of thepolymer backbone contributed by the diamine is either not para-orientedwith respect to the amino groups, or contains a non-linear constituent.

As mentioned previously, the aromatic polyimide prepared frompyromellitic dianhydride and p-phenylene diamine has the desiredpara-ordered geometry. This polymer has attractive high temperatureproperties and low cost. Unfortunately, when prepared via the solublepolymic acid route, the utility of the polymer is limited because of thewater produced in the thermal cyclodehydration step.

We attempted to prepare a polyimide from pyromellitic dianhydride andp-phenylene diamine via the polyisoimide to polyimide route. We foundthat the polyisoimide prepared from these monomers was insoluble in allthe solvents tested.

Accordingly, it is an object of the present invention to provide solublepolyisoimides which can be thermally rearranged to para-orderedpolyimides.

It is another object of the present invention to provide a method forpreparing soluble polyisoimides.

Other objects and advantages of the present invention will be readilyapparent to those skilled in the art.

SUMMARY OF THE INVENTION

In accordance with the present invention, there are providedpolyisoimides having repeating units of the formula ##STR4## wherein Ris selected from the group consisting of ##STR5## wherein Me is --CH₃and wherein X is ##STR6##

The soluble polyisoimide I can be thermally rearranged to provide arigid rod polyimide having repeating units of the formula ##STR7##wherein R is as described previously.

The polyisoimide I is prepared by reacting pyromellitic dianhydride withan aromatic diamine having the formula H₂ N--R--NH₂, wherein R is asdescribed previously, to form the corresponding polyamic acid. Theresulting polyamic acid is then dehydrated to form the correspondingpolyisoimide.

Suitable diamines include, for example, 2-phenoxy-1,4-diaminobenzenedihydrochloride, 2-(3-phenoxy phenyleneoxy)-1,4-diaminobenzenedihydrochloride, 3,3'-bis-phenosy-p-benzidine, 3,3'-bis(3-phenoxyphenyleneoxy)-p-benzidine, and 2,6,2',6'-tetramethyl-p-benzidine.

Preparation of the substituted p-phenylene diamine monomers is describedin our co-pending application Ser. No. 07/249,574, filed of even date,which is incorporated herein by reference. The tetramethyl-p-benzidinemonomer is commercially available. Preparation of the remainingsubstituted p-benzidine monomers is described in our co-pendingapplication Ser. No., 07/249,621, filed of even date, wich isincorporated herein by reference.

The reaction between the carboxylic acid dianhydride and the diamine ispreferably carried out in the presence of a solvent. It has been foundthat dimethylacetamide (DMAC) is a preferred solvent, although othersolvents, such as tetrahydrofuran (THF) or dioxane, may also beemployed.

The temperature at which the reaction is carried out is not critical anddepends, to some extent, on the particular reactants being used. Bestresults are usually obtained when the reaction temperature is maintainedbelow about 100° C. Higher temperatures can cause the resulting polyamicacid to cyclize to the corresponding ionide.

After the reaction has been completed, the product is in the form of apolyamic acid which can be converted to the corresponding isoimidepolymer through the use of a cyclization as dehydrating agent. Thetemperature at which cyclization is carried out is preferably atemperature in the range of about 0° C., to about 75° C., morepreferably about 0° C., to about 30° C. The dehydrating agent should beone which is soluble with a polymic acid in the reaction mixture, iscapable of cyclizing a polyamic acid at the aforesaid reducedtemperatures, does not adversely affect the solubility of any of thereactants in the reaction mixture and does not promote undesirable sideeffects. Suitable dehydrating agents include N,N'-dicyclohexylcarbodiimide (DCC), trifluoro-acetic anhydride, and the like.

The polyisoimides of the present invention are soluble in a variety ofsolvents, including DMAC, THF and the like.

The polyisoimides of the present invention are used to produce curedresins having high strength and temperature characteristics as well aslow void contents. Curing is effected by heating the polyisoimides attemperatures ranging from about 200° C. to about 370° C.

The following examples illustrate the invention:

EXAMPLE I

A 500-ml, three-necked, round-bottom flask was fitted with a mechanicalstirrer and nitrogen inlet/outlet. The flask was charged (undernitrogen) with 3,3',5,5'-tetramethyl p-benzidine (3.04219 g. 12.657mmol), pyromellitic dianhydride (2.7604 g, 12.6570 mmol) and 58.02 g offreshly distilled DMAC. After stirring at room temperature for 2 h, allof the starting materials dissolved and significant increase in thereaction mixture viscosity was noted. The reaction mixture was stirred(at room temperature) for an additional 12 h, then diluted with 246.95 gof anhydrous DMAC and 1.45 g of anhydrous lithium chloride. The mixturewas stirred for 1 h, after which all lithium chloride had dissolved. Tothis solution. DCC (4.933 g, 23.91 mmol) was added with stirring causinga rapid color change (dark red). The reaction mixture was stirred atroom temperature for 12 h, poured into 2400 ml of fresh isopropanol, andstirred for 2 h. A bright red-orange precipitate was collected on acourse fritted funnel (keeping the material covered with alcohol),washed with three 200-ml portions of fresh isoproganol and finallywashed with three 200-ml portions of anhydrous benzene. The final flurryof benzene and product was freeze dried at 0.1-mm Hg for 72 h. Yield:5.82 g (100.3%) of a bright red-orange fibrous solid.

EXAMPLE II

The procedure of Example I was generally followed using3,3'-phenoxy-p-benzidine.

EXAMPLE III

The procedure of Example I was generally followed using 3,3'-(3-phenoxyphenyleneoxy)-p-benzidine.

EXAMPLE IV

A 100-ml resin kettle was fitted with a four-necked ground-glass top,mechanical stirrer, and nitrogen inlet/outlet. The kettle was chargedwith 2-(3-phenoxyphenyleneoxy)-1,4-diaminobenzen dihydrochloride (0.6252g, 1.712 mmol), pyromellitic dianhydride (0.3734 g, 1.712 mmol), lithiumcarbonate (0.1130 g, 1.883 mmol) and 9.02 of freshly distilled DMAC.After stirring at room temperature for a few minutes, evolution of gaswas noted and continued for 1/2 h. The light-yellow reaction mixture wasstirred at room temperature for 12 h, diluted with 43.50 g of anhydrousDMAC, and stirred for an additional hour. To the solution, DCC (0.7205g, 3.492 mmol) was added with stirring, causing a rapid color change(dark red). The reaction mixture was stirred at room temperature for 12h, poured into 500 ml of fresh isopropanol and stirred for 2 h. A brightred-orange precipitate was collected on a medium fritted glass funnel,dried briefly under vacuum, redissolved in 50 ml of THF. andreprecipitated in 500 ml of isopropanol. The collected (suctionfiltration) red-orange solid weiqhed 0.86 g (98%) after drying at 40°C., for 48 hours.

EXAMPLE V

The procedure of Example IV was generally followed using2-phenoxy-1,4-diamino-benzene dihydrochloride.

Various modifications may be made without departing from the spirit ofthe invention or the scope of the appended claims.

We claim:
 1. A polyisoimide having repeating units of the formula##STR8## wherein R is selected from the group consisting of ##STR9##wherein Me is --CH₃ and wherein X is ##STR10##
 2. The polyisoimide ofclaim 1 wherein R is ##STR11##
 3. The polyisoimide of claim 1 wherein Ris ##STR12##
 4. The polyisoimide of claim 3 wherein X is phenoxy.
 5. Thepolyisoimide of claim 3 wherein X is phenoxyphenyleneoxy.
 6. Thepolyisoimide of claim 1 wherein R is ##STR13##
 7. The polyisoimide ofclaim 6 wherein X is phenoxy.
 8. The polyisoimide of claim 6 wherein Xis phenoxyphenyleneoxy.